This invention relates generally to a method and apparatus for passively shimming a magnet to correct deviations from a desired magnetic field. Methods and apparatus of the present invention are particularly useful in conjunction with magnetic resonance imaging (MRI) apparatus for correcting magnetic field inhomogeneities resulting from manufacturing tolerances. Although the examples cited herein involve MRI apparatus and superconducting magnets, the usefulness of the invention is not limited solely to such apparatus or magnets.
A high uniform magnetic field is useful for using magnetic resonance image (MRI) and nuclear magnetic resonance (NMR) systems as a medical device or a chemical/biological device. At least some popular MRI systems currently available utilize a superconducting magnet that creates a uniform field in a defined image volume of about 0.5 Tesla to about 9.4 Tesla in a pre-determined space (i.e., an imaging volume, or volume of interest).
Some types of materials can be made superconducting by placement in an extremely cold environment, such as a cryostat or pressure vessel containing cryogenic material. In the case of a magnet coil, such as those used in an MRI apparatus, extreme cold is used to make the magnet coils superconducting. In such a state, an initial current produced in the coil continues to flow even after power is removed. Superconducting magnets find wide application in the field of MRI due to their high magnetic field strength.
For proper operation of known MRI apparatus, the magnetic field of the magnet must be uniform in a defined image volume within a specified tolerance, often about 10 ppm. To compensate for inhomogeneities, MRI magnets, including superconducting magnets, are often shimmed utilizing various arrangements of correction coils and/or passive ferromagnetic shim materials.
MRI magnets are not immune to field inhomogeneity due to coil deformation and coil misalignment. Known superconducting magnet designs have been directed at minimizing such inhomogeneity during the design stage. To remove inhomogeneity that remains after the manufacturing cycle due to manufacturing tolerances, environmental effects, and/or design restrictions, passive shim systems have been used.
Various shimming methods for MRI magnets are known. At least one known method utilizes a combination of correction coils and passive ferromagnetic shims. The superconducting magnet is adjusted at the factory utilizing these coils and shims to provide a homogeneous magnetic field in the imaging bore of the magnet, which is also referred to herein as a “volume of interest.” Passive shims are positioned between a warm imaging bore and a gradient coil. As a result, in known MRI systems, it is difficult to access and adjust or change passive shims after the gradient coil is installed while minimizing the profile or space occupied by the shim assembly. However, due to magnetic material in the vicinity of the magnet at the installation site, it is frequently necessary to reshim the magnet to provide a specified field homogeneity.
Known passive shimming systems are difficult to adjust on-site. Thus, on-site adjustment has frequently been limited to varying the current through the correction coils. However, it is expensive to provide correction coils, associated circuitry and leads for this purpose.
Referring to FIGS. 1 and 2, at least one known MRI configuration provides passive shimming for a superconducting magnet using iron chips or similar ferromagnetic material before, after, or together with superconducting shimming. FIG. 1 is a schematic of the magnet with a removed shim tray, while FIG. 2 depicts the loading and unloading of the shim trays. Typically, the basic steps include (a) taking a field map by either using a field probe or a field camera, (b) calculating the field inhomogeneity and harmonics distribution, (c) calculating quantity and distribution of iron chips for passive shimming, (d) adding the iron chips to the shim trays outside of the magnet one-by-one according to the calculated results, (f) loading the shim trays into magnet bore, and (g) mapping the magnetic field again to verify the shimming effectiveness.
It is quite normal for a magnet to have a few shimming iterations to achieve the stringent uniformity requirement. The iteration process requires pulling out previously installed shim trays in order to modify (e.g., add/remove) iron shim chips, and then re-load these shim trays into the magnet again. Further, operators for the process have to deal with the magnetic interactive force, as well as having to manually load and unload shim chips. The process for passive shimming is a costly and labor-intensive process. In addition, the process is prone to error because of the manual loading and unloading shim chips.
In addition, current passive shimming calculations after field mapping includes a quantumize process. The quantumize process is needed as a result of the chips used in conventional passive shimming being categorized by a few manageable quantized or discrete sizes by either weight or dimension. The quantumize process lowers the accuracy (e.g., introducing errors) and increases the complexity of the shim chip distribution, hence increasing the cost.
Thus, there is a particular need for an MRI shimming method and system which provide improved magnetic field imaging homogeneity and yet which are uncomplex, eliminate errors associated with manual loading and unloading, and increase accuracy by providing flexibility over current discretely sized shims.